Hydrocarbons II: Substitution and Addition Reactions of Alkanes, Alkenes, and Alkynes

Hydrocarbons II

Overview

This study guide covers the chemical reactivity of hydrocarbons, focusing on the substitution reactions of alkanes and the addition reactions of alkenes and alkynes. It explains key mechanisms, regioselectivity rules (Markovnikov and anti-Markovnikov), and provides examples and theoretical context for each reaction type.

Classification of Hydrocarbons and Their Reactions

Concept Map

• Alkanes (Saturated): Undergo substitution reactions, primarily free radical substitution.

• Alkenes/Alkynes (Unsaturated): Undergo addition reactions, including hydrogenation, halogenation, hydrohalogenation, halohydrin formation, and hydration.

• Markovnikov's Rule: Governs regioselectivity in addition reactions of unsymmetrical alkenes and alkynes.

Substitution Reactions of Alkanes

General Features

• Involves two reactants exchanging parts to form two new products.

• Alkanes are generally inert and require heat or high-intensity light to initiate reactions.

• Most common mechanism: Free Radical Substitution.

Free Radical Substitution Mechanism

• Initiation: Formation of radicals, typically by homolytic cleavage of Cl2 under UV light or heat. $Cl_2 \xrightarrow{hv} 2Cl\cdot$

• Propagation: Radicals react with alkanes to form new radicals and products. $Cl\cdot + CH_4 \rightarrow CH_3\cdot + HCl$ $CH_3\cdot + Cl_2 \rightarrow CH_3Cl + Cl\cdot$

• Termination: Radicals combine to form stable molecules, ending the chain reaction. $Cl\cdot + Cl\cdot \rightarrow Cl_2$ $CH_3\cdot + Cl\cdot \rightarrow CH_3Cl$ $CH_3\cdot + CH_3\cdot \rightarrow C_2H_6$

Substitution Products of Alkanes

• Example: Chlorination of methane produces a series of products depending on the amount of Cl2 used.

  Reactants	Products
  CH4 + Cl2	CH3Cl + HCl
  CH3Cl + Cl2	CH2Cl2 + HCl
  CH2Cl2 + Cl2	CHCl3 + HCl
  CHCl3 + Cl2	CCl4 + HCl

Addition Reactions of Alkenes and Alkynes

General Features

• Alkenes and alkynes are more reactive due to the presence of double or triple bonds.

• Unsaturated compounds are electron-rich and act as nucleophiles.

• Addition reactions convert pi bonds to sigma bonds, increasing saturation.

Types of Addition Reactions

• Hydrogenation (H2): Addition of hydrogen across double/triple bonds, converting alkenes/alkynes to alkanes. Catalysts: Pd, Pt, Ni, Rh (often supported on charcoal). $RCH=CH_2 + H_2 \xrightarrow{Pd/C} RCH_2CH_3$

• Halogenation (X2): Addition of halogens (Cl2, Br2) to form dihalides. $CH_2=CH_2 + Br_2 \rightarrow BrCH_2CH_2Br$

• Hydrohalogenation (HX): Addition of hydrogen halides (HCl, HBr, HI) to form alkyl halides. $CH_2=CH_2 + HBr \rightarrow CH_3CH_2Br$

• Hydration: Addition of water (H2O) in the presence of acid to form alcohols. $CH_2=CH_2 + H_2O \xrightarrow{H_2SO_4} CH_3CH_2OH$

• Halohydrin Formation: Addition of X2 and H2O to form halohydrins.

Markovnikov's and Anti-Markovnikov's Rule

Markovnikov's Rule

• Predicts regioselectivity in electrophilic addition reactions of unsymmetrical alkenes/alkynes.

• Rule: The hydrogen atom of the reagent adds to the carbon of the multiple bond that already has the greater number of hydrogen atoms ("the rich get richer").

• Example: Addition of HBr to propene yields 2-bromopropane as the major product.

Theoretical Explanation

• Formation of carbocation intermediates; stability determines product distribution.

• Carbocation stability order: tertiary > secondary > primary.

  Carbocation Type	Relative Stability
  Tertiary (3°)	Most stable
  Secondary (2°)	Intermediate
  Primary (1°)	Least stable

• Lower activation energy for formation of more stable carbocation leads to Markovnikov product.

Anti-Markovnikov's Rule

• Observed in the presence of peroxides during addition of HBr to alkenes.

• Hydrogen adds to the carbon with fewer hydrogens (opposite of Markovnikov's rule).

Carbocation Rearrangement

Hydride and Alkyl Shifts

• Carbocations can rearrange via 1,2-hydride or 1,2-alkyl shifts to form more stable carbocations.

• Example: 1,2-hydride shift in 3-methyl-1-butene forms a more stable secondary carbocation.

Mechanisms of Addition Reactions

Electrophilic Addition of HX to Alkenes

• Step 1: Alkene pi electrons attack HX, forming a carbocation intermediate.

• Step 2: Halide ion attacks carbocation, yielding the alkyl halide.

Hydrogenation (Syn Addition)

• Both hydrogens add to the same face of the double bond (syn addition).

• Example: Hydrogenation of 1,2-dimethylcyclohexene yields cis-1,2-dimethylcyclohexane.

Halogenation (Anti Addition)

• Halogens add to opposite faces of the double bond (anti addition).

• Used as a test for alkenes: disappearance of bromine color indicates reaction.

• Alkanes do not react with bromine in the dark.

Hydration of Alkynes

• Water adds to alkynes in the presence of acid (H2SO4) and mercuric ion catalyst.

• Internal alkynes yield ketones; terminal alkynes require Hg2+ catalyst.

• Mechanism involves formation of an enol intermediate, which tautomerizes to a ketone.

Mechanism Steps for Hydration of Alkynes

1. Protonation of alkyne to generate carbocation.

2. Nucleophilic attack by water.

3. Deprotonation to form enol.

4. Reprotonation and tautomerization to yield ketone.

Practice Questions

• Q1: The most typical reaction of simple alkenes is electrophilic addition.

• Q2: Product from addition of Cl2 to 1-butene is 1,2-dichlorobutane.

• Q3: Suitable reagents for converting (CH3)2CHCH=CH2 to (CH3)2CHCH(OH)CH2Br include Br2, H2O.

• Q4: Compound that reacts rapidly with Br2 in the dark is isobutene (an alkene).

• Q5: Major product from 1-heptyne + 2 HBr is 1,1-dibromoheptane (Markovnikov addition).

• Q6: Carbocation stability order: II > I > III (tertiary > secondary > primary).

Summary Table: Key Reaction Types

Hydrocarbon Type	Reaction Type	Mechanism	Key Features
Alkanes	Substitution	Free Radical	Requires heat/light, inert, chain reaction
Alkenes/Alkynes	Addition	Electrophilic/Nucleophilic	Regioselectivity (Markovnikov/anti-Markovnikov), syn/anti addition

References

• Carey, F.A. (2008) Organic Chemistry 7th ed. McGraw Hill

• McMurry, J. (2008) Organic Chemistry 7th ed. Thomson Brooks Cole

• Bruice, P.Y. (2017) Organic Chemistry 8th ed. Prentice Hall International

• Bruice, P.Y. (2016) Essential Organic Chemistry: Study Guide and Solution Manual 3rd ed. Harlow: Pearson, cop.

• Brown W.H., Poon, T. (2016) Introduction to Organic Chemistry 6th ed. Hoboken, NJ: Wiley. (ebook)

Additional info: Academic context and explanations have been expanded for clarity and completeness. Mechanisms and tables have been reconstructed for study purposes.